Styrene is a monomer used in the manufacture of many plastics. Styrene is commonly produced by making ethylbenzene, which is then dehydrogenated to produce styrene. Ethylbenzene is typically formed by one or more aromatic conversion processes involving the alkylation of benzene.
Aromatic conversion processes, which are typically carried out utilizing a molecular sieve type catalyst, are well known in the chemical processing industry. Such aromatic conversion processes include the alkylation of aromatic compounds such as benzene with ethylene to produce alkyl aromatics such as ethylbenzene. Typically an alkylation reactor, which can produce a mixture of monoalkyl and polyalkyl benzenes, will be coupled with a transalkylation reactor for the conversion of polyalkyl benzenes to monoalkyl benzenes. The transalkylation process is operated under conditions to cause disproportionation of the polyalkylated aromatic fraction, which can produce a product having an enhanced ethylbenzene content and reduced polyalkylated content. When both alkylation and transalkylation processes are used, two separate reactors, each with its own catalyst, can be employed for each of the processes.
Ethylene is obtained predominantly from the thermal cracking of hydrocarbons, such as ethane, propane, butane, or naphtha. Ethylene can also be produced and recovered from various refinery processes. Thermal cracking and separation technologies for the production of relatively pure ethylene can account for a significant portion of the total ethylbenzene production costs.
Benzene can be obtained from the hydrodealkylation of toluene that involves heating a mixture of toluene with excess hydrogen to elevated temperatures (for example 500° C. to 600° C.) in the presence of a catalyst. Under these conditions, toluene can undergo dealkylation according to the chemical equation: C6H5CH3+H2→C6H6+CH4. This reaction requires energy input and as can be seen from the above equation, produces methane as a byproduct, which is typically separated and may be used as heating fuel for the process.
Other known process include the alkylation of toluene to produce styrene and ethylbenzene. In this alkylation process, various aluminosilicate catalysts are utilized to react methanol and toluene to produce styrene and ethylbenzene. However, such processes have been characterized by having very low yields in addition to having very low selectivity to styrene and ethylbenzene.
Additionally, it is known that the alkylation of toluene with methanol to produce styrene and ethylbenzene is a dehydrogenation reaction, which occurs efficiently at low pressures. Conversely, at high pressure, hydrogenation may occur, and the produced styrene may be hydrogenated to ethylbenzene. This hydrogenation reaction is undesirable in the production of styrene. Furthermore, it is known that pressure drop over the catalyst and the time that the reactants are in contact with the catalyst, commonly referred to as the “contact time,” are linearly related. As the length of contact time increases in the reactor, the likelihood of undesirable reactions, such as the hydrogenation of styrene to ethylbenzene, increases.
In view of the above, it would be desirable to have a process of producing styrene and/or ethylbenzene that does not rely on thermal crackers and expensive separation technologies as a source of ethylene. It would further be desirable to avoid the process of converting toluene to benzene with its inherent expense and loss of a carbon atom to form methane. It would be desirable to produce styrene without the use of benzene and ethylene as feedstreams. It would also be desirable to produce styrene at low pressure and with minimal contact time to increase the efficiency of the production of styrene and to reduce undesirable reactions, such as hydrogenation of styrene to ethylbenzene. Furthermore, it is desirable to have a reactor system able to achieve a high yield and selectivity to styrene and ethylbenzene.